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In a technologically advanced future, an elite human soldier takes command of a prototype star ship and works to defend the galaxy from danger.

The future is here: take control of Commander Shepard as the Galaxy's ultimate protector, who must save everything from the Universe's greatest threat: The Reapers. Mass Effect is a new frontier for gaming; it boldly goes where few have dared to go before.—Johnny-the-Film-Sentinel-2187

The game is set in the Milky Way galaxy in the year 2183. Interstellar travel is possible through the use of "Mass Relays", technology left behind by an ancient race known as the Protheans. A governing body called the Citadel Council, made up of several alien races, maintains order in the galaxy. Humanity is a relatively new member of this galactic community.

You play as Commander Shepard, a human soldier tasked with investigating a Prothean artifact on a human colony. The mission goes awry when the colony is attacked by a hostile alien race known as the Geth, led by a rogue Turian Spectre (an elite agent of the Citadel Council) named Saren Arterius.

During the attack, Shepard comes into contact with the Prothean artifact, which triggers visions of a terrifying threat: the Reapers, a race of sentient machines that periodically return to the galaxy to wipe out all advanced organic life.

After the attack on Eden Prime and the visions from the Prothean beacon, Shepard is appointed the first human Spectre, tasked by the Citadel Council to bring Saren to justice. This gives Shepard authority and resources to pursue Saren across the galaxy.

Shepard's investigation takes them to various locations, including:

The Citadel: The central hub of galactic civilization, where Shepard interacts with the Council and gathers information. Therum: A planet where Shepard rescues the Asari scientist Liara T'Soni, who becomes a valuable member of the squad and an expert on Prothean technology. Feros: A colony world under attack by the Thorian, a massive plant-like creature with mind control abilities. Here, Shepard uncovers more clues about Saren's plans and the Reapers. Noveria: A corporate-controlled world where Shepard investigates Saren's dealings with a synthetic intelligence called the Rachni.

Along the way, Shepard recruits a diverse squad of companions, each with their own skills and backstories:

Garrus Vakarian: A Turian former C-Sec officer seeking justice. Wrex: A Krogan mercenary with a troubled past. Tali'Zorah nar Rayya: A young Quarian on her Pilgrimage, seeking valuable technology for her people.

As Shepard pursues Saren, they discover the full extent of his plan:

Saren is searching for the "Conduit," a Mass Relay that leads to the Reapers' hiding place in dark space. He has allied himself with the Geth and is using their army to attack the Citadel. Saren has been indoctrinated by a Reaper called Sovereign, which he believes is a god that will bring about a new age.

The climax of the game takes place on the Citadel. Saren launches a full-scale attack with his Geth army, attempting to activate the Conduit and allow the Reapers to return to the galaxy. Shepard and their squad fight their way through the Citadel, facing waves of Geth and Saren's forces.

Shepard finally confronts Saren aboard Sovereign. Saren, realizing he has been manipulated by the Reapers, breaks free from their indoctrination but is ultimately defeated. Sovereign then attempts to take control of the Citadel, but Shepard, with the help of the Citadel fleet, manages to destroy it.

With Sovereign destroyed and Saren defeated, the immediate threat to the galaxy is averted. However, Shepard warns the Citadel Council that the Reapers are still a grave danger and will return. The game ends with the galaxy preparing for the inevitable Reaper invasion, setting the stage for the sequels.

Synopsis thanks: Marco Caruso